[Identity] Reorder Dockerfile contents
Summary: Addressing a comment from @jimpo in a previous diff. We should install rustfmt immediately after setting the user.
Test Plan: Build the docker image
Reviewers: jimpo, atul
Reviewed By: atul
Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh, jimpo
Differential Revision: https://phabricator.ashoat.com/D3818